Say you want to bring your nation to war with another nation - how would you go about doing this? As far as I'm aware there are no in game mechanics for this, so what would you do in RP to declare and wage war and declare peace?

The answer to your question is in your statement. RP (Role-play). As there is no game mechanic to declare war on certain nations, more-so to wage war on certain nations, the most effective way to work around that, is through Role-play. I would give you some examples of some ways in which war and role-play come into play in this game. I would also refer you to the game rule where it pertains to role-play and war.